A non-oriented electrical steel sheet is mainly used for a device that converts electrical energy into mechanical energy and requires superior magnetic characteristics to achieve high efficiency.
The magnetic characteristics may include an iron loss and a magnetic flux density, when the iron loss is low, energy may be less lost during the energy conversion process, while when the magnetic flux density is high, power is more generated with small electrical energy, and accordingly, a non-oriented electrical steel sheet having a low iron loss and a high magnetic flux density may increase energy efficiency of a motor.
In particular, since a high quality non-oriented electrical steel sheet recently used to operate a motor for an environmentally-friendly vehicle is used for a high speed rotation, a high-frequency iron loss is important to reduce, and herein, the high-frequency iron loss generally indicates an iron loss at a frequency of 400 Hz or higher, and in order to reduce this high-frequency iron loss, resistivity of a material is important to increase.
The magnetic characteristics of the non-oriented electrical steel sheet may be increased by generally adding Si as an alloy element. When the resistivity is increased through the addition of Si, the high-frequency iron loss may be preferably decreased, but workability as well as the magnetic flux density may be deteriorated, and so when the Si is added in an amount of greater than or equal to 3.5%, cold-rolling may be difficult to perform.
Accordingly, an attempt to adding a resistivity increase element such as Al, Mn, and the like other than the Si has been made. The addition of these elements may reduce the iron loss but deteriorate the magnetic flux density due to their entire alloy amount increases and cause difficulties in performing the cold rolling due to increased hardness of the material and deteriorated workability. In addition, the Al and the Mn are bonded with impurities inevitably present in the steel sheet and minutely precipitate nitride, sulfide, or the like and thus may rather deteriorate the iron loss.
In order to improve the magnetic characteristics of the non-oriented electrical steel sheet, high purification of steel is also very important. The iron loss may be reduced by managing the impurities in an extremely low level during a steelmaking process and thus minimizing an inclusion in a final product. However, the high purification of steel may not significantly improve the magnetic flux density but deteriorate steelmaking workability and increase a cost.
The magnetic characteristics of the non-oriented electrical steel sheet have an influence by a texture. When the non-oriented electrical steel sheet has a texture having a high parallel orientation fraction of a {001} plane to the plate surface of the non-oriented electrical steel sheet but a low parallel orientation fraction of a {111} plane to the plate surface of crystal orientations, excellent magnetic characteristics may be obtained.
Various methods of improving the magnetic characteristics by controlling the texture have been suggested. Japanese Patent No. 2004-197217 suggested a method of growing a grain size into greater than or equal to 400 μm after hot rolled plate-annealing and then, cold-rolling and recrystallization-annealing.
Japanese Patent No. 1996-088114 suggested a method of developing an advantageous texture for magnetic characteristics through twice cold-rollings including intermediate-annealing. However, these methods of improving a texture have a problem of extremely deteriorating productivity or increasing a cost when applied to an actual manufacture process.
On the other hand, a method of improving the texture through addition of a small amount of a grain boundary segregation element has been suggested in all sorts of documents. However, the present inventor found out that the texture and the magnetic characteristics were not almost improved through an experiment of adding the element within the range provided in the documents.